Hey Guys,

Just had an interesting question, I have a 2004 quad cab with a hemi and running the factory 17 inch wheels that came with the off road package, i believe 275 70 17, now I am looking at switching to the 05-08 stock 20 inch wheels for the 'summer' time here in wisconsin just for a little more bling and i like the way the wheels look. i am looking at running just the factory tire size which i think is a 275/60/20. Now i have seen threads about losing mpgs and power and etc. As i have the the superchips 3865 and can change tire sizes, will i really see that much loss with the AWD and even upgrading to a 20 inch wheel? Does the tune adjust itself somehow to not show that much loss? i am actually just running the 87 tune on it now, with upgrading wheels and tires would it make more sense to put the performance tune on it?

Thanks again for the help

Steve

 

In what way do you think adjusting the speedo for the tire size change will prevent you from physically losing MPGs? It just means your speedo will be correct and the MPG reading on the overhead will be more accurate.

The stock 20s are considerably heavier than the 17s and the 275/60/20s are about an inch taller than the 275/70/17s which will result in a bit more rolling resistance (not to mention a decrease in towing capacity).

Swapping to the Perf tune over the 87 should help a good bit with the resulting power loss and may even make up for some of the lost MPGs, but remember you'll now be paying 20-30 cents a gallon more for gas.

Guy down the street from me swapped his stock 20s after his wife slid the a$$ end off a culvert and destroyed one of the rims. Dealer wanted some stupid crazy amount like $485 for a replacement, so he opted for aftermarket 17s (aluminum) and gained about 3 mpg, with about the same diameter tire. So that may give you a general idea as to what you may lose in MPGs...

 

The difference in tire diameter between the stock 17" and the stock 20" is around 2.55% or .85". This would make your speedo too slow by 2.167%: 58.4mph at an actual 60mph. Fixing the tire size with the SC programmer only fixes your speedometer readout, it does nothing for your tune. At the same width and less than an inch taller, I cannot see this having a palpable effect on your MPG. However, what could certainly have an effect on your mileage, is the unsprung weight of the new wheels/tires. I do not have any figures regarding the weight of either 17" or 20" stock wheels, but that's where I would start, followed by the weight of the tires I would be mounting on said wheels.


**edit**
Looks like Hammer beat me to it o.ob
Something else to consider: I don't have solid numbers, but I believe the stock aluminum 20's from the '06-08 (like the one's on my truck in my sig) are lighter than the earlier 3rd gen 20's.

 

OK that makes sense, i guess i was thinking that playing around with the tuner and putting the performance tune on it i could save some of the MPG's that would be lost from and untuned truck, by getting back the power etc that i would be losing even though i would be adding the extra weight, I figured i was going to lose some MPG's if i end up getting them, i have not found a set in this area that are what i am looking for. but right now i am avging about 16.5 mpgs on the higway and 12.8 in the city, atleast that is what the overhead says. So i was just really seeing if there was anyway the tuner could offset that even though i was adding a little more weight and such. I would probably just mount a set of the Wrangler HP's that came stock as they would only be on for the summer months i figure i do not need to get anything to aggressive as i can save those for the 17's and the winter time.

 

So it looks like ill be adding 32-40 lbs going to the 20/s, so its actually about 288 total pounds for the 17 inch wheels with the stock 275/70/17 GY Wranglers roughly 47 lbs per tire and roughly 25 pounds per wheel, then about 80 lbs per wheel and tire package for the 20's roughly 43 lbs for the stock GY's and 37 lbs per wheel.. that was going off of tire racks stats for each OE tire in there respective sizes, and then a couple of threads i found on-line.

 

Another thing you should consider is there was a reason that truck came with 17" wheels. The off-road package would be pretty useless if it came with 20s. Very little sidewall flex with a 33" tire on 20" wheels. If you use that truck as intended from the factory, you won't really be able to air down or have useable sidewall flex if you take it off-road in the summer months...

 

Use this ...

http://www.discounttire.com/dtcs/infoTireMath.dos

 

Thanks for all the information, guess its not as simple as adding a set of wheels anymore.. :-) but that answered everything i need!..